This invention relates to a time division communication system having modular, plug-in peripheral port circuit packs that serve as communication interfaces to a variety of peripheral devices and more particularly to such a system in which each port circuit pack may have a variable number of time slots assigned thereto for any given period of time.
Traditional time division communication systems utilizing plug-in port circuit packs have time slots assigned to each plug-in slot and therefore to each installed port circuit pack on a permanent basis such that, for example, communications to or from a certain peripheral device can occur only during the preassigned time slots. This arrangement has the inherent limitation that a port circuit pack may not simultaneously serve more peripheral devices than the number of preassigned time slots would allow. Such a traditional system would require the permanent assignment to the port circuit of the maximum number of time slots usable by the peripheral devices even if some or all of the peripheral devices were idle.
To the extent that the total number of time slots in a practical communication system is finite and affects the economic cost of the system, such a traditional system would be, in this way, limited in the number of peripheral devices it could serve. Moreover, each modular plug-in port circuit pack would be constrained by the number of preassigned time slots in the number of peripheral devices it individually could serve.
Since different types of peripheral devices require different amounts of port circuitry, it is natural to be able to serve more of certain types than others from a corresponding type port circuit pack. If it is desired to be able to flexibly equip plug-in slots with port circuit pack types of varying densities, the number of preassigned time slots must correspond to the densest circuit types. The less dense types would then carry a penalty of inefficient time slot usage with a corresponding penalty in system capacity and economy. Using such an arrangement, it also would be impossible to take advantage of technological advances that might permit increased circuit density and therefore more peripheral devices per port circuit pack.
Similarly, technological advances may create the opportunity for new peripheral devices that require an increased number of time slots per device as compared with earlier devices. A traditional system might not be able to serve such new devices from a single port circuit pack if the number of required time slots exceeded the number preassigned to each port circuit pack.